If you are knackered and spent after 41 miles then
you are not eating and drinking properly. I have trashed myself in less than 41 hilly miles on several occasions when I have not eaten or drunk enough -
classic example - but I have ridden 200+ kms (125+ miles) many times and felt fine when I was more careful with my food and drink intake.
My speeds on rides of 40+ miles have depended on my fitness, the hilliness of the route, the weather conditions, which bike I was riding, how much effort I felt like making and who I was riding with, but would be
in the range 6 - 18 mph, occasionally 20+ mph in a hard-working group of strong riders.